Power conversion device

ABSTRACT

A power converter includes an arm in which a plurality of converter cells are connected in series, each of the converter cells including at least two switching elements, a power storage element and a pair of output terminals. A control device controls the power converter. The converter cell includes a switch to have the converter cell bypassed. When the control device senses failure of the converter cell, it has the failed converter cell bypassed, estimates an output voltage lost by bypassing the failed converter cell, and has a normal converter cell supply the estimated output voltage of the failed converter cell.

TECHNICAL FIELD

The present invention relates to a power conversion device.

BACKGROUND ART

A modular multilevel converter (which will be referred to as an MMC converter below) in which a plurality of unit converters (which will be referred to as converter cells below) are cascaded can readily address a high voltage by increasing converter cells. The modular multilevel converter has widely been applied to power transmission and distribution systems as a high-capacity static var compensator or an alternating-current (AC)-direct-current (DC) power conversion device for high-voltage DC power transmission. The converter cell includes a plurality of switching elements and a power storage element (which may be referred to as a capacitor). Even when a converter cell fails in the MMC converter, the MMC converter can continue operating by bypassing the failed converter cell.

PTL 1 describes an operation method without requiring adjustment of a modulation factor of each arm by adjusting the number of operating converter cells in each arm in accordance with the number of failures in an arm largest in number of failed converter cells and increasing a capacitor voltage.

CITATION LIST Patent Literature

-   PTL 1: Japanese Patent Laying-Open No. 2017-143619

SUMMARY OF INVENTION Technical Problem

PTL 1, however, does not consider a harmonic component in an arm current that increases after bypass of the failed converter cell.

An output voltage from a converter cell in the MMC converter contains a harmonic component such as a switching frequency component, an integer-order component thereof, and a sideband wave component thereof (which will simply be referred to as a harmonic component below), in addition to a frequency component included in an arm voltage command value such as a fundamental wave component on an AC output side (simply a fundamental wave component below) and a DC component. The MMC converter adopting phase shift pulse width modulation (PWM) cancels a harmonic component in an output voltage from each converter cell by equally shifting a PWM reference phase for each converter cell in each arm. A switching frequency of the output voltage from the arm can thus be made higher.

When a converter cell fails and the failed converter cell is bypassed, however, an output voltage from the failed converter cell becomes zero. Since an amount of shift of the PWM reference phase is thus no longer equal, the harmonic component in the output voltage from each converter cell cannot be canceled. Consequently, the harmonic component in the output voltage from each converter cell remains in the output voltage from the arm. Energy of the harmonic component is thus concentrated in some converter cells. Consequently, capacitor voltages of some converter cells deviate from a protection level and the MMC converter may stop operating for a protection purpose.

Therefore, an object of the present invention is to provide a power conversion device capable of suppressing a harmonic component in an output voltage from an arm that increases after bypass of a failed converter cell.

Solution to Problem

A power conversion device according to the present invention includes a power converter including an arm in which a plurality of converter cells are connected in series, each of the plurality of converter cells including at least two switching elements, a power storage element, and a pair of output terminals, and a control device to control the power converter. The converter cell includes a switch to have the converter cell bypassed. When the control device senses failure of a converter cell, the control device has a failed converter cell bypassed, estimates an output voltage lost by bypass of the failed converter cell, and has a normal converter cell supply the estimated output voltage of the failed converter cell.

Advantageous Effects of Invention

According to the present invention, a harmonic component in an arm current that increases after bypass of a failed converter cell can be suppressed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic configuration diagram of a power conversion device 1 in an embodiment.

FIG. 2 (a) and (b) is a diagram showing a configuration of a converter cell 7 that makes up a power converter 2.

FIG. 3 is a diagram showing an internal configuration of a control device 3 in a first embodiment.

FIG. 4 is a diagram showing a configuration of a basic controller 502.

FIG. 5 is a diagram showing a configuration of an arm controller 503.

FIG. 6 is a diagram showing a configuration of an individual cell controller 202.

FIG. 7 is a diagram showing a configuration of a cell additional voltage calculator 203.

FIG. 8 is a diagram showing an exemplary hardware configuration of control device 3.

FIG. 9 is a diagram showing a configuration of control device 3 in a second embodiment.

FIG. 10 is a diagram showing a configuration of a cell balancing circulating current controller 610.

FIG. 11 is a diagram showing a configuration of an arm voltage command generator 601 in the second embodiment.

FIG. 12 is a diagram showing a configuration of control device 3 in a third embodiment.

FIG. 13 is a diagram showing a configuration of a power conversion device 1A in a fourth embodiment.

FIG. 14 is a diagram showing a configuration of a part of a power conversion device 1B.

FIG. 15 is a diagram showing a configuration of a part of a power conversion device 1C.

FIG. 16 is a diagram showing a configuration of a basic controller 502A in a modification.

DESCRIPTION OF EMBODIMENTS

Embodiments will be described below with reference to the drawings.

First Embodiment

(Overall Configuration of Power Conversion Device)

FIG. 1 is a schematic configuration diagram of a power conversion device 1 in an embodiment. Referring to FIG. 1, power conversion device 1 is configured of a modular multilevel converter which includes multiple converter cells connected in series. Note that the “converter cell” is also called a “sub-module,” SM, or a “unit converter.” Power conversion device 1 converts power between a DC circuit 14 and an AC circuit 12. Power conversion device 1 includes a power converter 2 and a control device 3.

Power converter 2 includes multiple leg circuits 4 u, 4 v, 4 w (will be described as a leg circuit 4 when referred to collectively or when referring to any leg circuit) which are connected in parallel between a positive DC terminal (i.e., a high-potential-side DC terminal) Np and a negative DC terminal (i.e., a low-potential-side DC terminal) Nn.

Leg circuit 4 is provided for each of multiple phases constituting an alternating current. Leg circuit 4 is connected between AC circuit 12 and DC circuit 14, and converts power between the circuits. AC circuit 12 shown in FIG. 1 is a three-phase AC circuit which includes three leg circuits 4 u, 4 v, 4 w corresponding to a U phase, a V phase, and a W phase, respectively.

AC input terminals Nu, Nv, Nw provided for respective leg circuits 4 u, 4 v, 4 w are connected to AC circuit 12 via a transformer 13. AC circuit 12 is, for example, an AC power system which includes an AC power supply, etc. For ease of illustration, FIG. 1 does not show the connection between AC input terminals Nv, Nw and transformer 13.

High-potential-side DC terminal Np and low-potential-side DC terminal Nn, which are connected in common to each leg circuit 4, are connected to DC circuit 14. DC circuit 14 is, for example, a DC power system, including a DC power grid, or a DC terminal of other power conversion devices. In the latter case, a BTB (Back To Back) system for connecting different AC power systems having different rated frequencies is formed by coupling two power conversion devices.

AC input terminals Nu, Nv, Nw may be connected to AC circuit 12 via an interconnection reactor, instead of transformer 13 in FIG. 1. Furthermore, instead of AC input terminals Nu, Nv, Nw, a primary winding may be provided for each of leg circuits 4 u, 4 v, 4 w, and leg circuits 4 u, 4 v, 4 w may be connected to transformer 13 or an interconnection reactor in an AC manner via a secondary winding magnetically coupled to the primary winding. In this case, the primary winding may be reactors 8A, 8B described below. In other words, leg circuit 4 is electrically (i.e., a DC manner or an AC manner) connected to AC circuit 12 via the connector provided for each of leg circuits 4 u, 4 v, 4 w, such as AC input terminals Nu, Nv, Nw or the above primary winding.

Leg circuit 4 u includes an upper arm 5 from high-potential-side DC terminal Np to AC input terminal Nu, and a lower arm 6 from low-potential-side DC terminal Nn to AC input terminal Nu. AC input terminal Nu, which is the point of connection between upper arm 5 and lower arm 6, is connected to transformer 13. High-potential-side DC terminal Np and low-potential-side DC terminal Nn are connected to DC circuit 14. Leg circuits 4 v, 4 w have the same configuration as leg circuit 4 u, and leg circuit 4 u will thus be representatively described below.

Upper arm 5 includes multiple cascade-connected converter cells 7 and reactor 8A. Multiple converter cells 7 and reactor 8A are connected in series. Similarly, lower arm 6 includes multiple cascade-connected converter cells 7 and reactor 8B. Multiple converter cells 7 and reactor 8B are connected in series. In the description below, the number of converter cells 7 included in each of upper arm 5 and lower arm 6 is set to Ncell. Ncell is set to Ncell≥2.

Reactor 8A may be inserted anywhere in upper arm 5 of leg circuit 4 u. Reactor 8B may be inserted anywhere in lower arm 6 of leg circuit 4 u. Multiple reactors 8A and multiple reactors 8B may be present. The reactors may have different inductance values. Furthermore, only reactor 8A of upper arm 5 may be provided, or only reactor 8B of lower arm 6 may be provided.

Reactors 8A, 8B are provided to prevent a rapid increase of a fault current in the event of a fault of AC circuit 12 or DC circuit 14, for example. However, reactors 8A, 8B having excessive inductance values result in reduced efficiency of power converter 2. Accordingly, preferably, all the switching elements of each converter cell 7 are stopped (turned off) as soon as possible in the event of a fault.

As detectors for measuring the electrical quantities (current, voltage, etc.) for use in the control, power conversion device 1 further includes an AC voltage detector 10, an AC current detector 16, DC voltage detectors 11A, 11B, and arm current detectors 9A, 9B. Arm current detectors 9A, 9B are provided for each leg circuit 4. Signals detected by these detectors are input to control device 3.

Note that, for ease of illustration, in FIG. 1, some of signal lines for the signals input from the detectors to control device 3 and signal lines for the signals input/output to/from control device 3 and each converter cell 7 are depicted collectively, but they are, in practice, provided for each detector and each converter cell 7. The signal lines may be provided separately for transmission and reception of the signals between each converter cell 7 and control device 3. For example, an optical fiber may be adopted as the signal line.

In the following, each detector is described in detail.

AC voltage detector 10 detects a U-phase AC voltage Vacu, a V-phase AC voltage Vacv, and a W-phase AC voltage Vacw of AC circuit 12. In the description below, Vacu, Vacv, and Vacw are collectively denoted as Vac.

AC current detector 16 detects a U-phase AC current Iacu, a V-phase AC current lacy, and a W-phase AC current Iacw of AC circuit 12. In the description below, Iacu, lacy, and Iacw are collectively denoted as Iac.

DC voltage detector 11A detects a DC voltage Vdcp of high-potential-side DC terminal Np connected to DC circuit 14. DC voltage detector 11B detects a DC voltage Vdcn of low-potential-side DC terminal Nn connected to DC circuit 14. A difference between DC voltage Vdcp and DC voltage Vdcn is defined as a DC voltage Vdc.

Arm current detectors 9A and 9B, included in leg circuit 4 u for U phase, respectively detect an upper arm current Ipu flowing through upper arm 5 and a lower arm current Inu flowing through lower arm 6. Arm current detectors 9A and 9B, included in leg circuit 4 v for V phase, detect an upper arm current Ipv and a lower arm current Inv, respectively. Arm current detectors 9A and 9B, included in leg circuit 4 w for W phase, detect an upper arm current Ipw and a lower arm current Inw, respectively. In the description below, upper arm currents Ipu, Ipv, and Ipw are collectively denoted as an upper arm current Iarmp, lower arm currents Inu, Inv, and Inw are collectively denoted as a lower arm current Iarmn, and upper arm current Iarmp and lower arm current Iarmn are collectively denoted as Iarm.

(Converter Cell)

FIGS. 2 (a) and (b) is a diagram showing a configuration of converter cell 7 that makes up power converter 2.

Converter cell 7 shown in FIG. 2 (a) has a circuit configuration called a half-bridge configuration. This converter cell 7 includes a serial body formed by connecting two switching elements 31 p and 31 n to each other in series, a power storage element 32, a bypass switch 34, and a voltage detector 33. The serial body and power storage element 32 are connected in parallel.

Opposing terminals of switching element 31 n are defined as input and output terminals P1 and P2. A voltage across ends of power storage element 32 and a zero voltage are provided as a result of switching operations by switching elements 31 p and 31 n. For example, when switching element 31 p is turned on and switching element 31 n is turned off, the voltage across ends of power storage element 32 is provided. When switching element 31 p is turned off and switching element 31 n is turned on, the zero voltage is provided.

Bypass switch 34 is connected between input and output terminals P1 and P2. By turning on bypass switch 34, converter cell 7 is short-circuited. As converter cell 7 is short-circuited, switching elements 31 p and 31 n included in converter cell 7 are protected against an overcurrent produced at the time of a fault.

Voltage detector 33 detects a voltage Vc across ends of power storage element 32.

Converter cell 7 shown in FIG. 2 (b) has a circuit configuration called a full-bridge configuration. This converter cell 7 includes a first serial body formed by connecting two switching elements 31 p 1 and 31 n 1 to each other in series, a second serial body formed by connecting two switching elements 31 p 2 and 31 n 2 to each other in series, power storage element 32, bypass switch 34, and voltage detector 33. The first serial body, the second serial body, and power storage element 32 are connected in parallel.

A point intermediate between switching element 31 p 1 and switching element 31 n 1 and a point intermediate between switching element 31 p 2 and switching element 31 n 2 are defined as input and output terminals P1 and P2 of converter cell 7. The voltage across ends of power storage element 32 or the zero voltage is provided as a result of switching operations by switching elements 31 p 1, 31 n 1, 31 p 2, and 31 n 2.

Bypass switch 34 is connected between input and output terminals P1 and P2. By turning on bypass switch 34, converter cell 7 is short-circuited. As converter cell 7 is short-circuited, each element included in converter cell 7 is protected against an overcurrent produced at the time of a fault.

Voltage detector 33 detects voltage Vc across ends of power storage element 32.

In FIGS. 2 (a) and (b), switching elements 31 p, 31 n, 31 p 1, 31 n 1, 31 p 2, and 31 n 2 are configured, for example, by connection of a freewheeling diode (FWD) in anti-parallel to a self-arc-extinguishing semiconductor switching element such as an insulated gate bipolar transistor (IGBT) and a gate commutated turn-off (GCT) thyristor.

In FIGS. 2 (a) and (b), a capacitor such as a film capacitor is mainly employed as power storage element 32. Power storage element 32 may be referred to as a capacitor in the description below.

An example in which converter cell 7 has the half-bridge cell configuration shown in FIG. 2 (a), a semiconductor switching element is employed as the switching element, and a capacitor is employed as the power storage element will be described below by way of example. Converter cell 7 included in power converter 2, however, may have the full-bridge configuration shown in FIG. 2 (b). A converter cell in a configuration other than the configuration shown above, such as a converter cell to which a circuit configuration called a clamped double cell is applied, may be employed, and the switching element and the power storage element are not limited to the above either.

(Control Device)

FIG. 3 is a diagram showing an internal configuration of control device 3 in a first embodiment.

Control device 3 includes a switching controller 501 and a bypass controller 510.

Switching controller 501 controls on and off of switching elements 31 p and 31 n in converter cell 7.

When bypass controller 510 senses failure of converter cell 7 within the arm, it protects failed converter cell 7 within the arm against an overcurrent by turning on bypass switch 34 in failed converter cell 7.

Switching controller 501 includes a U-phase basic controller 502U, a U-phase upper arm controller 503UP, a U-phase lower arm controller 503UN, a V-phase basic controller 502V, a V-phase upper arm controller 503VP, a V-phase lower arm controller 503VN, a W-phase basic controller 502W, a W-phase upper arm controller 503WP, and a W-phase lower arm controller 503WN.

In the description below, U-phase basic controller 502U, V-phase basic controller 502V, and W-phase basic controller 502W are collectively denoted as a basic controller 502. U-phase upper arm controller 503UP, U-phase lower arm controller 503UN, V-phase upper arm controller 503VP, V-phase lower arm controller 503VN, W-phase upper arm controller 503WP, and W-phase lower arm controller 503WN are collectively denoted as an arm controller 503.

FIG. 4 is a diagram showing a configuration of basic controller 502.

Basic controller 502 includes an arm voltage command generator 601 and a capacitor voltage command generator 602.

Arm voltage command generator 601 calculates an arm voltage command value krefp for the upper arm and an arm voltage command value krefn for the lower arm. In the description below, krefp and krefn are collectively denoted as kref.

Capacitor voltage command generator 602 calculates a capacitor command voltage value Vcrefp for capacitors 32 in N converter cells 7 included in the upper arm. Capacitor voltage command generator 602 calculates a capacitor command voltage value Vcrefn for capacitors 32 in N converter cells 7 included in the lower arm. For example, an average voltage of capacitors 32 in converter cells 7 in the upper arm is defined as capacitor command voltage value Vcrefp and an average voltage of capacitors 32 in converter cells 7 in the lower arm is defined as capacitor command voltage value Vcrefn. In the description below, Vcrefp and Vcrefn are collectively denoted as Vcref.

Arm voltage command generator 601 includes an AC current controller 603, a circulating current calculator 604, a circulating current controller 605, and a command distributor 606.

AC current controller 603 calculates an AC control command value Vcp such that a difference between detected AC current Iac and a set AC current command value Iacref is set to 0.

Circulating current calculator 604 calculates a circulating current Iz that flows through one leg circuit 4 based on arm current Iarmp in the upper arm and arm current Iarmn in the lower arm. The circulating current is a current that circulates among a plurality of leg circuits 4. For example, circulating current Iz that flows through one leg circuit 4 can be calculated in accordance with an expression below.

Idc=(Ipu+Ipv+Ipw+Inu+Inv+Inw)/2  (1)

Iz=(Iarmp+Iarmn)/2−Idc/3  (2)

Circulating current controller 605 calculates a circulation control command value Vzp for controlling circulating current Iz to follow a set circulating current command value Izref such as 0.

Command distributor 606 receives AC control command value Vcp, circulation control command value Vzp, a DC voltage command value Vdcref, a neutral point voltage Vsn, and AC voltage Vac. Since an AC side of power converter 2 is connected to AC circuit 12 with transformer 13 being interposed, neutral point voltage Vsn can be calculated based on a voltage of a DC power supply of DC circuit 14. DC voltage command value Vdcref may be provided under DC output control or may be set to a constant value.

Based on these inputs, command distributor 606 calculates voltages to be supplied by the upper arm and the lower arm in a burden-sharing manner. Command distributor 606 determines arm voltage command value krefp for the upper arm and arm voltage command value krefn for the lower arm by subtracting voltage lowering due to an inductance component within the upper arm and the lower arm from respective calculated voltages.

Determined arm voltage command value krefp for the upper arm and arm voltage command value krefn for the lower arm serve as output voltage commands to control AC current Iac to follow AC current command value Iacref, to control circulating current Iz to follow circulating current command value Izref, to control DC voltage Vdc to follow DC voltage command value Vdcref, and to feedforward-control AC voltage Vac.

Basic controller 502 provides arm current lump of the upper arm, arm current Iarmn of the lower arm, arm voltage command value krefp for the upper arm, arm voltage command value krefn for the lower arm, capacitor command voltage value Vcrefp for the upper arm, and capacitor command voltage value Vcrefn for the lower arm.

FIG. 5 is a diagram showing a configuration of arm controller 503.

When arm controller 503 senses failure of converter cell 7, it has failed converter cell 7 bypassed and estimates an output voltage missed due to bypass of failed converter cell 7. Arm controller 503 has normal converter cell 7 provide the estimated output voltage of failed converter cell 7.

Arm controller 503 includes Ncell individual cell controllers 202 and a cell additional voltage calculator 203.

Individual cell controller 202 individually controls corresponding converter cell 7. Individual cell controller 202 receives arm voltage command value kref, arm current Iarm, and capacitor command voltage value Vcref from basic controller 502. Individual cell controller 202 receives a capacitor voltage Vc and a cell normality determination signal cn from corresponding converter cell 7. The cell normality determination signal is set to “1” when converter cell 7 is normal, and the cell normality determination signal is set to “0” when converter cell 7 has failed. Individual cell controller 202 receives a cell additional voltage signal dcvm from cell additional voltage calculator 203.

Individual cell controller 202 generates a gate signal ga for corresponding converter cell 7 and provides the gate signal to corresponding converter cell 7. Individual cell controller 202 generates a lost output voltage signal cvm for corresponding converter cell 7 and provides the lost output voltage signal to cell additional voltage calculator 203.

FIG. 6 is a diagram showing a configuration of individual cell controller 202.

Individual cell controller 202 includes an individual cell balance controller 2021, a PWM modulator 2022, signal switches 2023A, 2023B, 2023C, and 2023D, a cell output voltage estimator 2024, and an adder 2051.

Signal switch 2023A receives cell additional voltage signal dcvm provided from cell additional voltage calculator 203 and a zero signal representing a zero voltage. Signal switch 2023A provides a signal selected depending on cell normality determination signal cn. When converter cell 7 is normal, cell normality determination signal cn is set to “1” and cell additional voltage signal dcvm is provided. When converter cell 7 fails, cell normality determination signal cn is set to “0” and the zero signal is provided.

Individual cell balance controller 2021 provides an individual cell balance control output dkrefc such that capacitor voltage Vc matches with capacitor command voltage value Vcref based on capacitor command voltage value Vcref, capacitor voltage Vc of corresponding converter cell 7, and arm current Iarm. For example, individual cell balance controller 2021 can generate individual cell balance control output dkrefc based on a result of multiplication of a difference between Vcref and Vc by a gain K.

Signal switch 2023B receives individual cell balance control output dkrefc and a zero signal representing a zero voltage. Signal switch 2023B provides a signal selected depending on cell normality determination signal cn. When converter cell 7 is normal, cell normality determination signal cn is set to “1” and individual cell balance control output dkrefc is provided. When converter cell 7 fails, cell normality determination signal cn is set to “0” and the zero signal is provided.

Adder 2051 adds a signal provided from signal switch 2023A, a signal provided from signal switch 2023B, and arm voltage command value kref. A result of addition is provided as cell voltage command value krefc.

PWM modulator 2022 provides a PWM modulated signal by modulating cell voltage command value krefc in accordance with phase shift PWM with a carrier reference phase CRP and a dead time DT being defined as parameters. PWM modulator 2022 performs modulation depending on a configuration of converter cell 7. Depending on the configuration of converter cell 7, the number n of provided PWM modulated signals also increases or decreases. For example, in the case of a half-bridge cell, n is set to n=2, and in the case of a full-bridge cell, n is set to n=4.

When there are Ncell converter cells 7 in one arm, intervals among Ncell PWM carrier reference phases CRP within one arm become even by allocating phases different by 360°/Ncell to PWM modulators 2022 in converter cells 7 within one arm. Thus, a harmonic component in the output voltage from each converter cell 7 can be canceled and an equivalent switching frequency of the output voltage from one arm can be high. When converter cell 7 fails and the output voltage from failed converter cell 7 becomes zero, however, the harmonic component in the output voltage from each converter cell 7 cannot be canceled. Consequently, the harmonic component in the output voltage from each converter cell 7 remains in the output voltage from the arm.

Signal switch 2023C receives the PWM modulated signal and a zero signal representing a zero voltage. Signal switch 2023C provides a signal selected depending on cell normality determination signal cn. When converter cell 7 is normal, cell normality determination signal en is set to “1” and the PWM modulated signal is provided. When converter cell 7 fails, cell normality determination signal cn is set to “0” and the zero signal is provided. The signal provided from signal switch 2023C is sent to a gate driver for switching elements 31 p and 31 n in corresponding converter cell 7 as gate signal ga to control switching of switching elements 31 p and 31 n in corresponding converter cell 7.

Cell output voltage estimator 2024 receives the PWM modulated signal. Cell output voltage estimator 2024 estimates the output voltage from corresponding converter cell 7 when switching elements 31 p and 31 n are switched in response to the PWM modulated signal. Cell output voltage estimator 2024 provides an estimation signal pcv representing magnitude of the estimated cell output voltage. Specifically, cell output voltage estimator 2024 estimates the output voltage from corresponding converter cell 7 as the zero voltage or the capacitor voltage, depending on whether switching elements 31 p and 31 n in corresponding converter cell 7 are turned on or off in response to the PWM modulated signal. Specifically, cell output voltage estimator 2024 estimates the output voltage from corresponding converter cell 7 as capacitor voltage Vc (voltage across ends of power storage element 32) when switching element 31 p is turned on and switching element 31 n is turned off in response to the PWM modulated signal to switching element 31 p and the PWM modulated signal to switching element 31 n. Cell output voltage estimator 2024 estimates the output voltage from corresponding converter cell 7 as the zero voltage when switching element 31 p is turned off and switching element 31 n is turned on in response to the PWM modulated signal to switching element 31 p and the PWM modulated signal to switching element 31 n.

Though only the PWM modulated signal is assumed to be provided to cell output voltage estimator 2024 for the sake of brevity, arm current Iarm may be provided thereto. In this case, cell output voltage estimator 2024 can estimate the output voltage during a dead time period based on arm current Iarm. Specifically, when switching element 31 p is turned off and switching element 31 n is turned off in response to the PWM modulated signal to switching element 31 p and the PWM modulated signal to switching element 31 n, cell output voltage estimator 2024 estimates the output voltage from corresponding converter cell 7 in accordance with an orientation of arm current Iarm. The output voltage from corresponding converter cell 7 can thus highly accurately be estimated.

Signal switch 2023D receives estimation signal pcv and a zero signal representing magnitude of a zero voltage. Signal switch 2023D provides a signal selected depending on cell normality determination signal cn. When converter cell 7 is normal, cell normality determination signal cn is set to “1” and the zero signal is provided as lost output voltage signal cvm representing magnitude of an estimated value of the lost output voltage. When converter cell 7 fails, cell normality determination signal cn is set to “0” and estimation signal pcv is provided as lost output voltage signal cvm. Specifically, when converter cell 7 fails, individual cell controller 202 estimates based on the PWM modulated signal, a voltage that should have been provided from that converter cell 7 if converter cell 7 had been normal, and provides a result of estimation to cell additional voltage calculator 203.

FIG. 7 is a diagram showing a configuration of cell additional voltage calculator 203.

Cell additional voltage calculator 203 includes a lost output voltage addition unit 2031, a normal cell calculator 2032, and a divider 2033.

Lost output voltage addition unit 2031 adds lost output voltage signals cvm estimated and calculated in Ncell individual cell controllers 202 and provides a total lost output voltage signal ccvm representing the total of lost output voltages of at least one failed converter cell 7 in the arm.

Normal cell calculator 2032 adds Ncell cell normality determination signals cn. A result of addition represents the number of normal converter cells (the number of normal cells below) within the arm.

Divider 2033 divides total lost output voltage signal ccvm by the number sc of normal cells and provides cell additional voltage signal dcvm representing a result of division. Cell additional voltage signal dcvm represents magnitude of an additional output voltage allocated to one normal converter cell 7 within the arm. Cell additional voltage signal dcvm is sent to signal switch 2023A of individual cell controller 202.

According to the configuration, power conversion device 1 in the first embodiment can continue operating even when converter cell 7 fails.

(Operations by Control Device 3)

Operations by control device 3 in each of a case where all converter cells 7 are normal and a case where at least one converter cell 7 has failed will be described below.

(Operations when all Converter Cells 7 within Arm are Normal)

Operations by control device 3 when all converter cells 7 within the arm are normal will be described.

When all converter cells 7 within the arm are normal, cell normality determination signals cn for all converter cells 7 within the arm are set to “1”. Since the zero signal is provided from signal switch 2023D in all individual cell controllers 202 within the arm, all lost output voltage signals cvm are set to 0. Thus, cell additional voltage signal dcvm provided from cell additional voltage calculator 203 is also set to 0. In other words, when all converter cells 7 within the arm are normal, cell additional voltage signal dcvm is set to 0.

In individual cell controller 202, signal switch 2023B provides individual cell balance control output dkrefc provided from individual cell balance controller 2021. Signal switch 2023A provides cell additional voltage signal dcvm (=0) provided from cell additional voltage calculator 203.

Adder 2051 adds arm voltage command value kref, individual cell balance control output dkrefc, and cell additional voltage signal dcvm (=0) to obtain cell voltage command value krefc representing a result of addition.

PWM modulator 2022 performs PWM modulation on cell voltage command value krefc and provides the PWM modulated signal. Signal switch 2023C provides the PWM modulated signal. The PWM modulated signal provided from signal switch 2023C is sent to the gate driver for switching elements 31 p and 31 n in converter cell 7 as gate signal ga to control switching of switching elements 31 p and 31 n in converter cell 7.

(Operations when Failed Converter Cell 7 is Included within Arm)

Operations by control device 3 when failed converter cell 7 is included within the arm will be described.

When at least one failed converter cell 7 is included in the arm, cell normality determination signal cn for failed converter cell 7 is set to “0”. Thus, in individual cell controller 202 corresponding to failed converter cell 7, signal switch 2023D provides estimation signal pcv as lost output voltage signal cvm.

Lost output voltage signal cvm is provided to cell additional voltage calculator 203, and cell additional voltage signal dcvm representing magnitude of the additional output voltage allocated to one normal converter cell 7 within the arm is obtained.

(Operations by Normal Converter Cell)

In individual cell controller 202 in normal converter cell 7, signal switch 2023A provides cell additional voltage signal dcvm and signal switch 2023B selects and provides individual cell balance control output dkrefc.

Adder 2051 adds cell additional voltage signal dcvm, individual cell balance control output dkrefc, and arm voltage command value kref, and provides cell voltage command value krefc representing a result of addition.

PWM modulator 2022 performs PWM modulation on cell voltage command value krefc and provides the PWM modulated signal. Signal switch 2023C provides the PWM modulated signal. The PWM modulated signal provided from signal switch 2023C is sent to the gate driver for switching elements 31 p and 31 n in converter cell 7 as gate signal ga to control switching of switching elements 31 p and 31 n in converter cell 7.

(Operations by Failed Converter Cell) In individual cell controller 202 in failed converter cell 7, signal switches 2023A and 2023B both provide the zero signal, and hence cell voltage command value krefc provided from adder 2051 is equal to arm voltage command value kref.

PWM modulator 2022 performs PWM modulation on cell voltage command value krefc and provides the PWM modulated signal, whereas signal switch 2023C provides the zero signal. Therefore, gate signal ga is the zero signal. Switching elements 31 p and 31 n in converter cell 7 are thus turned off.

As a result of the operations above, the lost output voltage which should have been provided from failed converter cell 7 within the arm is included in the cell voltage command value for normal converter cell 7 within the arm. Therefore, the output voltage lost as a result of bypass of failed converter cell 7 is supplied from normal converter cell 7. Thus, a voltage harmonic that has been provided from failed converter cell 7 is vicariously supplied from normal converter cell 7 and a harmonic current that increases due to absence of a counter voltage can be suppressed. Consequently, power conversion device 1 can continue operating without protective suspension of power conversion device 1 due to deviation of capacitor voltages of some converter cells 7 from the protection level.

(First Modification of First Embodiment)

Though cell output voltage estimator 2024 estimates the output voltage of failed converter cell 7 based on the PWM modulated signal in the description above, a method of estimation is not limited. For example, cell output voltage estimator 2024 may estimate the output voltage of failed converter cell 7 based on a carrier reference phase of failed converter cell 7 and the cell voltage command value.

(Second Modification of First Embodiment)

Though corresponding individual cell controllers 202 of all normal converter cells 7 within the arm are described as calculating the cell voltage command value resulting from addition of the cell additional voltage in the description above, limitation thereto is not intended. Corresponding individual cell controllers 202 of some normal converter cells 7 within the arm may calculate the cell output voltage command value resulting from addition of the cell additional voltage so as to have some normal converter cells 7 within the arm supply a counter voltage. In this case, cell additional voltage calculator 203 calculates cell additional voltage signal dcvm by dividing total lost output voltage signal ccvm representing the sum of lost output voltages of failed converter cells 7 by the total number of some normal converter cells 7 within the arm. Cell additional voltage signal dcvm represents magnitude of the additional voltage allocated to one of some normal converter cells 7 within the arm.

(Hardware Configuration of Control Device 3)

FIG. 8 is a diagram showing an exemplary hardware configuration of control device 3.

Control device 3 is configured similarly to what is called a digital relay device. Control device 3 includes an analog-digital (AD) converter unit 530, an arithmetic processing unit 535, an input and output (IO) unit 543, and a settling-and-display unit 547.

In a stage preceding AD converter unit 530, a plurality of transformers (not shown) to convert input signals from arm current detectors 9A and 9B, AC voltage detector 10, AC current detector 16, DC voltage detectors 11A and 11B, and voltage detector 33 to a voltage level suitable for signal processing within control device 3 may be provided.

AD converter unit 530 includes an analog filter 531 and an AD converter 532. Analog filter 531 is a low-pass filter provided to remove an aliasing error in AD conversion. AD converter 532 converts the signal that has passed through analog filter 531 into a digital value.

While FIG. 8 representatively shows only one channel as the input to AD converter unit 530, AD converter unit 530, in practice, has a multiple-input configuration to receive the signals from the respective detectors. Accordingly, more specifically, AD converter unit 530 includes multiple analog filters 531, and a multiplexer (not shown) for selecting signals having passed through analog filters 531.

Arithmetic processing unit 535 includes a central processing unit (CPU) 536, a memory 537, bus interfaces 538, 539, and a bus 540 connecting these components. CPU 536 controls the entire operation of control device 3. Memory 537 is used as a primary storage for CPU 536. Furthermore, by including a nonvolatile memory, such as a flash memory, memory 537 stores programs, and settings values for the signal processing.

Note that arithmetic processing unit 535 may be configured of any circuit that has computing functionality, and is not limited to the example of FIG. 8. For example, arithmetic processing unit 535 may include multiple CPUs. Instead of the processor such as CPU, arithmetic processing unit 535 may be configured of at least one ASIC (Application Specific Integrated Circuit), or at least one FPGA (Field Programmable Gate Array). Alternatively, arithmetic processing unit 535 may be configured of any combination of the processor, ASIC, and FPGA.

IO unit 543 includes a communication circuit 544, a digital input circuit 545, and a digital output circuit 546. Communication circuit 544 generates an optical signal to be provided to each converter cell 7. The signal provided from communication circuit 544 is transmitted to converter cell 7 through an optical repeater 555. Digital input circuit 545 and digital output circuit 546 are interface circuits for communication between CPU 536 and external devices.

Settling-and-display unit 547 includes a touch panel 548 for inputting settling values and for display. Touch panel 548 is an input/output interface which is a combination of a visual display, such as a liquid crystal panel, and an input device, such as a touchpad. Touch panel 548 is connected to bus 540 via a bus interface 539.

Second Embodiment

It is when an AC output current and a DC output current from power converter 2 are low and an effective value of an arm current is small that there is a concern about imbalance in capacitor voltages in normal converter cells 7 due to increase in harmonic component of arm current Iarm when converter cell 7 fails.

Though individual cell balance control is sufficiently effective when the effective value of the arm current is large, it may not be sufficiently effective when the effective value of the arm current is small.

AC output power and DC output power, however, are determined by a higher-order command. Therefore, the AC output current and the DC output current cannot freely be set. Since a circulating current of a DC component and an AC fundamental wave component is basically used for controlling balance in average value of each phase of capacitor voltages of converter cells 7 or controlling balance in average value between the upper arm and the lower arm, there is no degree of freedom.

In a second embodiment, when the AC output current and the DC output current from power converter 2 are low to such an extent that the capacitor voltages of converter cells 7 within the arm are not balanced, a current at a frequency different from a frequency of a current provided from power converter 2 is circulated within power converter 2. Since the effective value of the arm current is thus larger, individual cell balance control is sufficiently effective and imbalance among converter cells 7 is rectified. The current at the frequency different from the frequency of the current provided from power converter 2 refers to a current other than a DC current and an AC current (a current of the fundamental wave) provided from AC circuit 12.

FIG. 9 is a diagram showing a configuration of control device 3 in the second embodiment.

Control device 3 in the second embodiment includes switching controller 501 and bypass controller 510 similarly to control device 3 in the first embodiment and includes a cell balancing circulating current controller 610.

When cell balancing circulating current controller 610 senses failure of converter cell 7 in any of the plurality of arms, it has a current circulated within power converter 2 in order to increase the effective value of arm current Iarm, the current having a frequency different from the frequency of the current provided from power converter 2.

FIG. 10 is a diagram showing a configuration of cell balancing circulating current controller 610.

Cell balancing circulating current controller 610 includes a first coordinate converter 611, a compensator 612, and a second coordinate converter 613.

First coordinate converter 611 converts circulating current components Izu, Izv, and Izw of three phases of U, V, and W onto a dq two-phase coordinate that rotates at a frequency θ different from the frequency of the current provided from power converter 2. Resultant Izd represents an effective component and Izq represents a reactive component, both of which represent a DC quantity.

Compensator 612 provides two-phase DC voltage command components Vzdref and Vzqref such that two-phase circulating current components Izd and Izq follow circulating current command components Izdref and Izqref converted to the two phases.

Second coordinate converter 613 converts two-phase DC voltage command components Vzdref and Vzqref calculated by compensator 612 to three-phase DC voltage command circulating current components Vdccu, Vdccv, and Vdccw. V DC voltage command circulating current components dccu, Vdccv, and Vdccw are sent to arm voltage command generators 601 of U-phase basic controller 502U, V-phase basic controller 502V, and W-phase basic controller 502W, respectively. In the description below, Vdccu, Vdccv, and Vdccw are collectively denoted as Vdcc.

FIG. 11 is a diagram showing a configuration of arm voltage command generator 601 in the second embodiment.

Command distributor 606 receives AC control command value Vcp, circulation control command value Vzp, DC voltage command value Vdcref, neutral point voltage Vsn, and AC voltage Vac as in the first embodiment and receives DC voltage command circulating current component Vdcc.

Command distributor 606 calculates based on these inputs, voltages to be supplied by the upper arm and the lower arm as in the first embodiment. Command distributor 606 determines arm voltage command value krefp for the upper arm and arm voltage command value krefn for the lower arm by subtracting voltage lowering due to an inductance component within the upper arm and the lower arm from the calculated voltages.

Since the effective value of arm current Iarm increases according to the present embodiment, the fundamental wave that makes up the cell voltage command value is larger. Consequently, individual cell balance control is sufficiently effective and imbalance among converter cells 7 is rectified.

Third Embodiment

In the present embodiment, transformer 13 is defined as a transformer variable in transformation ratio. The transformer variable in transformation ratio is implemented, for example, by a transformer with a tap switching function.

Since AC output power and DC output power are determined by a higher-order command in the second embodiment, the AC output current and the DC output current are described as not freely being set.

Power conversion device 1 is interconnected to AC circuit 12 with transformer 13 being interposed. Therefore, by varying the transformation ratio of transformer 13, AC output current Vac can be varied without affecting AC output power and DC output power. Since the effective value of arm current Iarm can thus be increased, imbalance among converter cells 7 can be rectified.

FIG. 12 is a diagram showing a configuration of control device 3 in a third embodiment.

Control device 3 in the third embodiment includes switching controller 501 and bypass controller 510 as in the first embodiment and includes a transformer controller 504.

When transformer controller 504 senses failure of converter cell 7 in any of the plurality of arms, it varies the transformation ratio of transformer 13 in order to increase the effective value of arm current Iarm. Specifically, the AC current that flows from AC circuit 12 to power conversion device 1 is increased by lowering a ratio N (V2/V1) between a voltage V1 on a side of AC circuit 12 of transformer 13 and a voltage V2 on a side of power conversion device 1. Since the effective value of arm current Iarm thus increases, the fundamental wave that makes up the cell voltage command value is larger. Consequently, influence of a harmonic can be lessened.

Fourth Embodiment

FIG. 13 is a diagram showing a configuration of a power conversion device 1A in a fourth embodiment.

Power conversion device 1A in the fourth embodiment is different from power conversion device 1 in the first embodiment in that each arm in a power converter 2A of power conversion device 1A in the fourth embodiment includes a redundant converter cell (RSM) 7 a. Redundant converter cell 7 a is similar in configuration to converter cell 7 shown in FIG. 2 (a) and (b).

Redundant converter cell 7 a in each arm is bypassed before occurrence of failure in any converter cell 7 within each arm. Therefore, during this period, redundant converter cell 7 a does not perform a conversion operation.

Bypass of redundant converter cell 7 a in each arm is canceled after occurrence of failure in any converter cell 7 within each arm. Therefore, during this period, redundant converter cell 7 a performs the conversion operation instead of converter cell 7 that has failed.

Before any converter cell 7 within the arm fails, bypass controller 510 turns on bypass switch 34 of redundant converter cell 7 a within that arm. After any converter cell 7 within the arm fails, bypass controller 510 turns off bypass switch 34 of redundant converter cell 7 a within that arm.

As set forth above, in the present embodiment, after any converter cell 7 within each arm fails, redundant converter cell 7 a performs the conversion operation instead of converter cell 7 that has failed. Thus, even when converter cell 7 fails, the number of converter cells that operate in one arm does not vary and a harmonic component in the output voltage from the converter cell can be canceled.

The number of redundant converter cells 7 a within the arm is not limited to one but a plurality of redundant converter cells may be provided. Redundant converter cell 7 a within the arm does not have to be fixed. A redundant converter cell may be selected every certain cycle, sequentially, or randomly, from among a plurality of converter cells that have not failed.

(Modification)

The present invention is not limited to the embodiments above but includes also modifications as below.

(1) Configuration of Power Converter 2

In the embodiment above, power converter 2 is in a configuration called a double star. Power converter 2 is mainly used as an AC-DC converter for high voltage direct current (HVDC) power transmission. Control of the power converter described in the embodiment above is also applicable to a power converter in another configuration.

FIG. 14 is a diagram showing a configuration of a part of a power conversion device 1B.

A power converter 2B of power conversion device 1B is in a configuration called a single delta. Power converter 2B is mainly used for a static var compensator.

FIG. 15 is a diagram showing a configuration of a part of a power conversion device 1C.

A power converter 2C of power conversion device 1C is in a configuration called a single star. Power converter 2C is also mainly used for a static var compensator.

A scheme for suppressing a harmonic produced due to failure of converter cell 7 as described in the embodiments above is effective also in power converters 2B and 2C. In power converter 2C including an output only on the AC side, a circulating current described in the second embodiment may be a current at a frequency other than a fundamental wave component. For example, a circulating current containing a DC component may flow.

(2) Though triangular wave comparison PWM modulation is described by way of example, sawtooth wave comparison PWM modulation in which carriers are sawtooth waves may be applicable and carriers are not restricted. In a case of space voltage vector PWM modulation as well, by adding a function to achieve a similar effect, a converter can continue operating also in case of failure in a converter cell.

(3) Control described in the embodiment above may intermittently operate when imbalance in capacitor voltage among converter cells 7 is aggravated.

(4) Converter Cell for Circulating Current Control

When an arm includes a common converter cell and a converter cell for controlling a circulating current, the configuration of the basic controller is different from that shown in FIG. 4.

FIG. 16 is a diagram showing a configuration of a basic controller 502A in a modification.

Basic controller 502A in FIG. 16 is different from basic controller 502 in FIG. 4 in that circulation control command value Vzp provided from basic controller 502A is not provided to a command distributor 606A.

Command distributor 606A receives AC control command value Vcp, DC voltage command value Vdcref, neutral point voltage Vsn, and AC voltage Vac. Based on these inputs, command distributor 606A calculates voltages to be supplied by the upper arm and the lower arm. Command distributor 606A determines arm voltage command value krefp for the upper arm and arm voltage command value krefn for the lower arm by subtracting voltage lowering due to an inductance component within the upper arm and the lower arm from the calculated voltages. A signal to control common converter cells within the arm is generated based on arm voltage command value krefp and arm voltage command value krefn for the lower arm, as described in the first embodiment.

A not-shown control block provides a PWM modulated signal to the converter cell for circulator control based on circulation control command value Vzp provided from basic controller 502A.

It should be understood that the embodiments disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims rather than the description above and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

REFERENCE SIGNS LIST

1, 1A, 1B, 1C power conversion device; 2, 2A, 2B, 2C power converter; 3 control device; 4 u, 4 v, 4 w leg circuit; 5 u, 5 v, 5 w upper arm; 6 u, 6 v, 6 w lower arm; 7 converter cell; 7 a redundant converter cell; 8A, 8B reactor; 9A, 9B arm current detector; 10 AC voltage detector; 11A, 11B DC voltage detector; 12 AC circuit; 13 transformer; 14 DC circuit; 16 AC current detector; 31 p, 31 n, 31 p 1, 31 n 1, 31 p 2, 31 n 2 switching element; 32 power storage element; 33 voltage detector; 34 bypass switch; 202 individual cell controller; 203 cell additional voltage calculator; 501 switching controller; 502, 502A basic controller; 502U U-phase basic controller; 502V V-phase basic controller; 502W W-phase basic controller; 503UP U-phase upper arm controller; 503UN U-phase lower arm controller; 503VP V-phase upper arm controller; 503VN V-phase lower arm controller; 503WP W-phase upper arm controller; 503WN W-phase lower arm controller; 504 transformer controller; 510 bypass controller; 530 AD converter unit; 531 analog filter; 532 AD converter; 535 arithmetic processing unit; 536 CPU; 537 memory; 538, 539 bus I/F; 540 bus; 543 IO unit; 544 communication circuit; 545 digital input circuit; 546 digital output circuit; 547 settling-and-display unit; 548 touch panel; 555 optical repeater; 601 arm voltage command generator; 602 capacitor voltage command generator; 603 AC current controller; 604 circulating current calculator; 605 circulating current controller; 606 command distributor; 610 cell balancing circulating current controller; 611 first coordinate converter; 612 compensator; 613 second coordinate converter; 2022 PWM modulator; 2023A, 2023B, 2023C, 2023D signal switch; 2024 cell output voltage estimator; 2031 lost output voltage addition unit; 2032 normal cell calculator; 2051 adder; Nn low-potential-side DC terminal; Np high-potential-side DC terminal; Nu, Nv, Nw AC input terminal 

1. A power conversion device comprising: a power converter including an arm in which a plurality of converter cells are connected in series, each of the plurality of converter cells including at least two switching elements, a power storage element, and a pair of output terminals; and a control device to control the power converter, the converter cell including a switch to have the converter cell bypassed, wherein when the control device senses failure of a converter cell, the control device has a failed converter cell bypassed, estimates an output voltage lost by bypassing the failed converter cell, and has a normal converter cell supply the estimated output voltage of the failed converter cell, the control device includes a basic controller to provide an arm voltage command value, and at least one individual cell controller to individually control a corresponding converter cell within the arm upon receiving the arm voltage command value, the individual cell controller corresponding to the failed converter cell estimates the output voltage lost by bypassing the failed converter cell, and the individual cell controller corresponding to the normal converter cell calculates a voltage command value for the corresponding converter cell based on the arm voltage command value and the output voltage lost by bypassing the failed converter cell.
 2. (canceled)
 3. The power conversion device according to claim 1, wherein the individual cell controller includes a PWM modulator to provide a PWM modulated signal upon receiving a cell voltage command for a corresponding converter cell, and a cell output voltage estimator to estimate an output voltage of the corresponding converter cell based on the PWM modulated signal.
 4. The power conversion device according to claim 3, wherein the cell output voltage estimator estimates the output voltage of the converter cell as a zero voltage or a voltage across ends of the power storage element, depending on whether the switching element is turned on or off in response to the PWM modulated signal.
 5. The power conversion device according to claim 3, wherein the individual cell controller includes a first signal switch to provide a signal indicating magnitude of the voltage estimated by the cell output voltage estimator when the corresponding converter cell has failed and to provide a signal indicating a zero voltage when the corresponding converter cell is normal.
 6. The power conversion device according to claim 5, wherein the control device includes a cell additional voltage calculator to provide a signal indicating magnitude of an additional output voltage allocated to one of all normal converter cells by dividing a sum of signals provided from the first signal switch of the at least one individual cell controller by a total number of all normal converter cells within the arm.
 7. The power conversion device according to claim 5, wherein the control device includes a cell additional voltage calculator to provide a signal indicating magnitude of an additional output voltage allocated to one of some normal converter cells by dividing a sum of signals provided from the first signal switch of the at least one individual cell controller by a total number of some normal converter cells within the arm.
 8. The power conversion device according to claim 6, wherein the individual cell controller includes a second signal switch to provide a signal indicating magnitude of a zero voltage when the corresponding converter cell has failed and to provide an output signal of the cell additional voltage calculator when the corresponding converter cell is normal, and an adder to add the arm voltage command value provided from the basic controller and an output from the second signal switch to send a result of addition to the PWM modulator as a cell voltage command value.
 9. The power conversion device according to claim 8, wherein the individual cell controller includes an individual cell balance controller to provide an individual cell balance control output such that a voltage from the power storage element of the corresponding converter cell matches with an average voltage of power storage elements in the converter cells within the arm, and the adder adds the arm voltage command value provided from the basic controller, the output from the second signal switch, and the individual cell balance control output to send a result of addition to the PWM modulator as the cell voltage command value.
 10. The power conversion device according to claim 1, wherein when the control device senses failure of a converter cell, the control device increases an effective value of an arm current that flows through the arm.
 11. The power conversion device according to claim 10, wherein the control device has a current circulated within the power converter for increasing the effective value of the arm current, the current containing a frequency component different from a frequency component of a current provided from the power converter.
 12. The power conversion device according to claim 10, comprising a transformer variable in transformation ratio, wherein the control device increases an AC output current by varying the transformation ratio for increasing the effective value of the arm current.
 13. The power conversion device according to claim 7, wherein the individual cell controller includes a second signal switch to provide a signal indicating magnitude of a zero voltage when the corresponding converter cell has failed and to provide an output signal of the cell additional voltage calculator when the corresponding converter cell is normal, and an adder to add the arm voltage command value provided from the basic controller and an output from the second signal switch to send a result of addition to the PWM modulator as a cell voltage command value. 